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United States Patent |
6,260,777
|
Seppanen
|
July 17, 2001
|
Method and equipment for manufacturing predetermined low bark content wood
chips and a high bark content fuel fraction from wood chips with bark
attached
Abstract
A method and equipment manufacturing predetermined low bark content wood
chips and a high bark content fuel fraction from wood chips with bark
attached, includes mechanical bark removal treatment, in which the bark is
removed from the chips and their size is reduced; pre-cleaning of the flow
of chips with bark attached, into a flow of chips with a bark content of
less than 10% and high bark content rejects, with the aid of pneumatic and
mechanical sieving; final cleaning of the aforesaid wood chip flow from
the pre-cleaning, with the aid of an optical separator, to create low bark
content wood chips and a fraction with a higher bark content; collection
of the rejects leaving the process, to create the aforementioned high bark
content fuel fraction; and seiving out of the flow of wood chips, before
the mechanical bark removal process, a predetermined thin fraction of the
chips, which bypasses the debarking process.
Inventors:
|
Seppanen; Veli (Jyvaskyla, FI)
|
Assignee:
|
Valtion teknillinen tutkimuskeskus (Espoo, FI)
|
Appl. No.:
|
068273 |
Filed:
|
April 30, 1998 |
PCT Filed:
|
November 1, 1996
|
PCT NO:
|
PCT/FI96/00586
|
371 Date:
|
April 30, 1998
|
102(e) Date:
|
April 30, 1998
|
PCT PUB.NO.:
|
WO97/16590 |
PCT PUB. Date:
|
May 9, 1997 |
Foreign Application Priority Data
Current U.S. Class: |
241/19; 241/24.2; 241/28; 241/81 |
Intern'l Class: |
B02C 019/12 |
Field of Search: |
241/28,81,19,24.2
|
References Cited
U.S. Patent Documents
4036440 | Jul., 1977 | Makila et al. | 241/28.
|
4332353 | Jun., 1982 | Lario et al. | 241/28.
|
5000390 | Mar., 1991 | Marrs | 241/28.
|
5137621 | Aug., 1992 | Brown | 241/81.
|
5577671 | Nov., 1996 | Sappanen et al. | 241/28.
|
5709257 | Jan., 1998 | Vinogradov et al.
| |
Foreign Patent Documents |
WO 93/25324 | Dec., 1993 | WO.
| |
Primary Examiner: Rosenbaum; Mark
Attorney, Agent or Firm: Fildes & Outland, P.C.
Claims
What is claimed is:
1. A method for manufacturing predetermined low bark content wood chips and
a high bark content fuel fraction from wood chips with bark attached,
comprising the steps of:
sieving the wood chips with bark attached into a predetermined thin wood
chip fraction and sieving overflow chips;
mechanically removing bark from the sieving overflow chips, thereby
producing smaller overflow chips, and
pre-cleaning, with the aid of pneumatic and mechanical sieving, the
predetermined thin wood chip fraction and the smaller overflow chips, into
a flow of chips with a bark content of less than 10% and a first fraction
of higher bark content rejects;
final cleaning of said flow of chips with a bark content of less than 10%,
with the aid of an optical sorter, to create low bark content wood chips
and a second fraction of higher bark content rejects;
collecting at least part of said first and second higher bark content
rejects to create said high bark content fuel fraction.
2. The method according to claim 1, characterized in that the step of
mechanically removing bark from a flow of chips having bark attached takes
place by means of a grinder, press-rollers, or a crusher.
3. The method according to claim 1, characterized in that the step of
mechanically removing bark from a flow of chips having bark attached takes
place using a a blade ring chipper, which cuts the wood chips into smaller
pieces, while removing the bark.
4. The method according to claim 1, characterized in that 20-100% of the
second fraction of higher bark content rejects obtained from the optical
sensor is directed to the fuel fraction, while 80-0% is returned to the
step of mechanically removing bark.
5. The method according to claim 4, characterized in that the optical
sensor includes a pneumatic conveyor and a division plate, by means of
which the flow of chips with a bark content of less than 10% is divided
first into said low bark content wood chips and said second fraction of
higher bark content rejects, said second fraction of higher bark content
rejects being further divided with the aid of the division plate directly
into a fuel fraction, with a remainder of said second fraction of higher
bark content rejects being returned to the step of mechanically removing
the bark.
6. The method according to claim 1, characterized in that it includes a
pre-separation step before the sieving step, in which oversized wood
chips, stones, and metals are removed from the wood chips with bark
attached.
7. The method according to claim 1, characterized in that the predetermined
thin wood chip fraction and smaller overflow chips are sieved using a
mechanical sieve, to remove sawdust as a third reject and by using a
pneumatic separator to remove light birch bark, leaves, needles, and other
airborne material as a fourth reject.
8. Apparatus for manufacturing wood chips with a low bark content from wood
chips with bark attached, said apparatus comprising:
a wood chip thickness sieve for separating the flow of wood chips with bark
attached into an outflow of a predetermined thin wood chip fraction and an
outflow of sieving overflow chips;
a mechanical bark removal device functionally integrated with said wood
chip thickness sieve;
pre-cleaning devices functionally integrated with said mechanical bark
removal device, said pre-cleaning devices comprising a mechanical and a
pneumatic separator and an optical sorter;
said apparatus further including a correction line functionally integrated
between said wood chip thickness sieve and said pre-cleaning devices to
receive said outflow of said predetermined thin wood chip fraction so that
said predetermined thin fraction bypasses said mechanical bark removal
devices and flows directly to the pre-cleaning devices.
9. Equipment according to claim 8, characterized in that the mechanical
bark removal devices consist of at least one of a blade ring chipper, a
grinder, a press roller, and a crusher.
Description
FIELD OF THE INVENTION
This invention relates to a method and equipment for manufacturing
predetermined low bark content wood chips and a high bark content fuel
fraction from wood chips with bark attached. More specifically the method
includes a mechanical bark removal process, pre-cleaning of wood chips
with bark attached, and final cleaning of the flow of wood chips, with the
aid of an optical separator.
BACKGROUND OF THE INVENTION
A wood bark manufacturing method is known from international patent
publication WO 93/25324. According to this, the wood chips with bark
attached are ground initially either with a special plate grinder, or with
a vibrating cone crusher, wherein the bark separates from the wood chips
and the size of the bark particles diminishes. Thereby, the bark can be
separated more easily during later separation stages. A grinder or a
vibrating cone crusher is followed by pneumatic separation, to separate
outer birch bark, after which sawdust is removed using a vibrating sieve.
The bark content has now dropped to considerably less than 10%, and the
final cleaning can be carried out using an optical sorter. According to
the patent, two optical sorters are used in series, but can, however, be
replaced by a single more powerful device. The accepted fraction from the
second optical sorter is led to grinding and the reject to the fuel
fraction.
Equipment that is essentially that described by the application has been
built in Kankaanpaa in Finland, the supplier being BMH Wood Technology Oy.
In the commercial equipment, a magnetic separator and a pneumatic
separator are used in the pre-cleaning to separate metal scrap and stones.
Sawdust is removed from the commercial equipment before grinding, and
after grinding the vibrating sieve is replaced by a so-called pocket-roll
sieve (a roller sieve developed especially for sieving sawdust). In the
commercial equipment, a good yield is achieved with a bark content of
about 1%, which is sufficiently clean for the manufacture of certain
grades of cellulose. The yield varies between 60-70%, depending on the
species of timber and other factors. If cleaner chips are wanted, the
yield drops, and correspondingly improves with a poorer level of
cleanliness.
SUMMARY OF THE INVENTION
An object of the present invention is to achieve a better yield than
previously at each corresponding level of cleanliness.
The characteristic features of the method for manufacturing predetermined
low bark content wood chips and a high bark content fuel fraction from
wood chips with bark attached, includes the steps of:
mechanical bark removal treatment, in which the bark is removed from the
chips and their size is reduced,
pre-cleaning of the flow of chips with bark attached, into a flow of chips
with a bark content of less than 10% and high bark content rejects, with
the aid of pneumatic and mechanical sieving,
final cleaning of the aforesaid wood chip flow from the pre-cleaning, with
the aid of an optical separator, to create low bark content wood chips and
a fraction with a higher bark content, and
collection of the rejects leaving the process, to create the aforementioned
high bark content fuel fraction, characterized in that before the
mechanical bark removal process, a predetermined thin fraction of the
chips, is sieved out of the flow of wood chips, and then bypasses the
debarking process.
The mechanical bark removal process may take place by means of a grinder,
press-rollers, or a crusher. Alternatively, the mechanical bark removal
takes place using a blade ring chipper, which cuts the wood chips into
smaller pieces, while removing the bark.
According to the method, 20-100% of the fraction with a higher bark content
obtained from the optical separator is directed to the fuel fraction, and
correspondingly 80-0% is returned to the mechanical bark removal process.
The optical separator includes a pneumatic conveyor and a division plate,
by means of which the flow of chips is divided first into the said wood
chips with a low bark content and wood chips with a higher bark content
and further the latter part is divided with the aid of the division plate
directly into a fuel fraction and the said part that is returned.
The method may include a pre-separation step before the thickness sieving
step, in which the oversized wood chips, stones, and metals are removed
from the wood chips with bark attached.
The flow of wood chips from the mechanical bark removal process and the
thin chip fraction from the thickness sieving may be sieved using a
mechanical sieve, to remove sawdust as a third reject and using a
pneumatic separator to remove light birch bark, leaves, needles, and other
airborne material as a fourth reject.
Equipment for manufacturing wood chips with a low bark content from wood
chips with bark attached, which equipment includes mechanical bark removal
devices and pre-separation devices consisting of a mechanical and a
pneumatic separator and an optical sorter. The equipment is characterized
in that it includes a wood chip thickness sieve located before the
mechanical bark removal devices and a correction line connected to the
exit of the thin fraction obtained, by means of which the thin fraction is
taken past the mechanical bark removal devices to the pre-cleaning
devices.
The mechanical bark removal devices may consist of at least one of a blade
ring chippers, a grinder, a press roller, and a crusher.
The distribution values of the wood chip fractions according to the
accompanying table are, to a great extent, the point of departure of the
invention:
Sieve Fraction Clean Wood Bark % of
gap remaining wood chips chips wood chips
mm in sieve % in fraction with bark with bark
0 5,03
2 16,73 79,03 1,96 3,61
4 37,94 89,98 2,72 8,57
6 25,37 85,54 7,15 16,78
8 8,01 74,54 14,83 19,98
10 6,92 69,00 25,51 18,30
It is surprising, that in the smallest fractions (sieve gap 4 mm, or less),
there is not much bark attached to the chips, whereas in chips above this
size the proportion of chips with bark attached is considerable. Because
mechanical processing in any event reduces the size of the wood chips
across the board, it is advantage s to separate this fairly clean part of
the wood chips and have it bypass the mechanical processing. There is bark
as such even in the smallest chips, but it is loose, and most of it leaves
during the separation of the sawdust, which has been moved so that it
takes place after the mechanical processing.
These and other features and advantages of the invention will be more fully
understood from the following detailed description of the invention taken
together with the accompanying drawings, which show schematically one
plant according to the invention and the method used in it.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 illustrates the method according to the invention, as a block
diagram; and
FIG. 2 illustrates equipment according to the invention, as a plant
diagram.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings in detail, in the diagram in FIG. 1, the wood
chips with bark attached can be considered as having been obtained by
conventional methods, using a known cutter. In the first stage of
processing, the wood chips are cleaned in pre-separation 3, 4, 5, which
comprises, among other things, magnetic devices for removing scrap iron
and a pneumatic separator 4, 4' for removing stones and sand. In
pre-separation, oversize wood chips are separated by means of disc sieve
5, and in practice are most advantageously led to the fuel fraction, but
if necessary they can be led, for example, to a crusher and then back to
pre-separation. The fraction accepted in pre-separation 3, 4, 5 is led to
thickness sieving 6, which is intended to separate the thin wood chip
fraction, generally less than 6 mm, from the part of the flow of wood
chips travelling through the thickness sieve. This part is led to the
following stage, i.e. past mechanical bark separation 7. Mechanical bark
separation can take place by means of either a plate grinder, a crusher,
or in a blade ring chipper, which has been shown to be the most
advantageous, because it does not defibrate the wood material
unnecessarily.
The mechanically processed fraction, together with the smaller fraction
obtained from the bypass line, is led to pocket-roll sieve 9, which
effectively removes the sawdust from the flow of material. After this, the
accepted fraction is led to a pneumatic separator, to remove light birch
bark, leaves, needles, and other easily airborne particles from the flow
of material. After this, the pre-cleaned wood chips already have a bark
content of less than 10%, and can be led to optical sorter 10, from which
wood chips with a bark content of about 1% can be obtained, even at a
large yield. Reject from the optical sorter is led through selection 15 in
a regulated manner either to the fuel fraction, or back to mechanical
processing. In order to prevent a so-called `mad cycle`, 20-100% is always
led to the fuel fraction, and only 80-0% is led to the mechanical
processing. Certain species of timber or other conditions will mean that
there is not necessarily any benefit from mechanical processing, in which
case it is better to lead the reject from the optical sorter directly to
the fuel fraction.
The plant diagram of FIG. 2, shows cutter 1 and reception funnel 2, in
which the wood chips are collected, or into which wood chips from external
sources are poured. From here the flow of wood chips is led through
magnetic separator 3 to disc sieve 5. Between these there is a pneumatic
separator 4', which removes stones and a second pneumatic separator 4,
which sucks light birch bark, leaves, and fine dust into the airflow, see
also stone store 3.1. A disc sieve 5 removes oversized pieces from the
wood chips, which are led to the fuel fraction 13, together with reject
from the pneumatic separator. Beneath the disc sieve there is a thickness
sieve 6, which separates the smallest fraction and the sawdust in it,
these being led to mechanical bark separation, going past grinder 7,
whereas the rest of this part is led to grinder 7. Experiments have,
however, shown that bark can be effectively separated by cutting the
chips. The mechanically processed chip fraction obtained from the plate
grinder and the thin fraction obtained from the bypass line 8 are led to
the Pocket-Roll sieve 9, which removes sawdust and fine bark material from
the flow of chips. After the Pocket-Roll sieve, there is a pneumatic
separator 11, which removes light birch bark, leaves, needles, and other
easily airborne material.
The accepted fraction is led to optical sorter 10, from which a good yield
of good quality cellulose wood chips 12 are obtained. The reject is led
through selection member 15, either to the fuel fraction 13 or to the
grinder 7. Using selection member 15, 0-100% can be taken from the flow of
materials into the fuel fraction. It is advantageous to take at least 20%,
to prevent a so-called `mad cycle` in the process. With certain grades of
wood chips part of the material would remain to circulate continuously
through the process, unless part of the reject is removed to the fuel
fraction. All of the removed rejects are led in the figure to fuel
fraction 13, by means of conveyor 16.
Because the optical sorter uses a pneumatic conveyor, the reject flow is
not homogeneous. A divider plate 18 set in the exit area can separate the
part with the greatest bark content from the rest of the reject, it then
being advantageously led directly to the fuel fraction with the aid of
conveyor 19. The figure shows this alternative by broken lines.
The pulverization of the wood is reduced with the aid of the method
according to the invention, which increases the cellulose chip yield from
the previous yield by 5-10 percentage units, while the fraction of
cellulose chips remaining in the 13 mm particle size perforated sieve
increases substantially (by 15-35 percentage units).
In equipment according to the invention, a thickness sieve is used to
separate the fraction with the desired value, 4-8 mm, for example, thinner
than 6 mm, which is led past the barking separation and the thicker wood
chips are fed to the mechanical bark separation. These fractions are
combined and the sawdust and light birch bark is sieved out and sorted
optically, the accepted fraction of which is clean wood chips and the
reject is returned to mechanical bark separation, in which the bark is
ground to a smaller size, so that it can be distinguished in the sawdust
discharge.
Improvement of the cellulose wood chip yield by 5 percentage units improves
the profitability of the plant by about FIM 20/m.sup.3, which represents
FIM 3 000 000 p.a. in a plant producing 150 000 m.sup.3 p.a.. The effect
of the chip size of cellulose wood chips on the sales price varies from
case to case, but at its greatest is about FIM 25/m.sup.3. Together these
correspond, in magnitude, to the operating and capital costs of the plant,
so that the economy of the method improves substantially.
Although the invention has been described by reference to a specific
embodiment, it should be understood that numerous changes may be made
within the spirit and scope of the inventive concepts described.
Accordingly, it is intended that the invention not be limited to the
described embodiment, but that it have the full scope defined by the
language of the following claims.
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